1. Field of the Invention
The present invention relates to cooking surfaces of glass-ceramic used on domestic kitchen ranges which have a heat source on the underside of the cooking surface, the heat energy being transmitted through the cooking surface to the upper side. The present invention relates specifically to a glass-ceramic plate having two layers, one of which transmits and another of which absorbs thermal radiations in the near infrared range.
2. Description of the Prior Art
For several years, glass-ceramics have been known which can be used in the manufacture of cooking surfaces. These various glass-ceramic cooking surfaces differ from each other in their radiation transmittance for visible rays as well as for radiation having wave lengths in the infrared area. The various glass-ceramic surfaces find application in combination with heating elements which function according to two different principles.
In one application, the heating element contacts the underside of the glass-ceramic and functions according to the principles of conductance between the heating and cooking surfaces. In other application, the heating element is spaced from the underside of the glass-ceramic surface and functions according to the principles of radiation. In both applications the heating element must have a temperature limitation imposed in order to minimize over-heating of the glass-ceramic which might result in a structural failure. This imposition of the temperature limitation establishes a theoretical maximum heat delivering capability for each given application.
It has long been thought that to achieve the best possible performance consistent with economy of operation, the makeup of the glass-ceramic plate should be selected so as to optimize the heat delivering capability. Especially in the application employing a heating element spaced from the underside of the glass-ceramic plate, the radiation transmittance value of the glass-ceramic plate in the infrared wave length area was thought to be determinative of the boiling time and efficiency.
Surprisingly, it has been discovered that the best cooking performance does not come from the use of a glass-ceramic plate with the highest radiation transmittance value. This surprising result is believed to be caused by partial reflection of thermal radiation by the cooking vessel back through the glass-ceramic plate to the area of the heating element. This, in turn, increases the temperature in the region of the heating element triggering the temperature limitation, thus reducing the amount of thermal radiation which the heating unit is permitted to emit.
An additional disadvantage of glass-ceramic plates with higher values for radiation transmittance lies in the fact that when used with transparent or translucent glass or glass-ceramic cookware, the food can easily burn since the radiation partially goes through the cooking surface and cookware bottom unhindered directly to the food. This occurs especially with high output radiation heating units which are used to make a fast boiling time possible in these ranges.
The typical glass-ceramics with high radiation transmittance values in the infrared range also have good transparency in the area of visible wave lengths. For this reason, these glass-ceramics are disadvantageous in combination with radiation heating sources since the brightly glowing heating elements shine through the plate to an undesirable extent.